Apparatus and method for automatic optimization of atrioventricular delay for an active medical device

ABSTRACT

A method for use by an active medical device includes using a stimulation device and an endocardial acceleration sensor to obtain a plurality of hemodynamic parameters associated with at least three atrioventricular delays. The method further includes using the plurality of hemodynamic parameters to find a second derivative associated with the atrioventricular delays. The method further includes using interpolation to estimate an atrioventricular delay which will reduce the second derivative associated with the atrioventricular delays. The method further includes using the estimated atrioventricular delay in a subsequent stimulation.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/030,002 entitled “Apparatus and Method for Automatic Optimization ofAtrioventricular Delay for an Active Medical Device” and filed Feb. 17,2011, which claims the benefit of French Application No. 1051120entitled “Active Implantable Medical Device Such As Pacemaker,Defibrillator and/or Resynchronizer with Automatic Optimization of theAtrioventricular Delay” and filed Feb. 17, 2010, both of which arehereby incorporated by reference in their entireties.

BACKGROUND

The present invention relates to “active implantable medical devices” asdefined by the 20 Jun. 1990 Directive 90/385/EEC of the Council of theEuropean Communities, and more particularly to those devices thatcontinuously monitor a patient's heart rhythm and if necessary deliverto the heart of the patient electrical pulses for stimulation,resynchronization and/or defibrillation in response to detection of acardiac rhythm disorder.

A typical implantable medical device runs a classic “dual chamber”operating mode in which the device monitors the ventricular activityafter an atrial event that is either spontaneous (i.e., P wave detectionof an atrial depolarization) or stimulated (i.e., application of an Apulse of atrial pacing). After detecting an atrial event, the devicestarts to count a delay period referred to as “atrio-ventricular delay”(AVD). If no spontaneous ventricular activity (R wave detection of aventricular depolarization) is detected at the expiry of an AVD, thedevice triggers stimulation of a ventricle (either left or rightventricle or both) by applying an electrical pulse for ventricularpacing.

The settings of the device are regularly reassessed to adjuststimulation parameters if necessary. The configuration and parametersettings for the stimulation therapy are appropriately modified as thepatient's clinical status evolves over time.

The standard technique for adjusting stimulation parameters includingthe AVD starts with the estimation of the characteristic delays of thesystole by echocardiography, especially the delay of opening of theaortic valve. However, this adjustment procedure should be implementedin hospitals and by qualified personnel. The procedure is long andcostly, and thus cannot be applied as often as it would be useful ornecessary without interfering with the patient's daily life despite thebeneficial effects of the stimulation therapy.

A “multisite” implantable device that has more than two stimulationsites makes the echocardiographic assessment even more difficult becauseit requires testing several successive pacing configurations (e.g.,selecting different sites and/or sequencing different stimulation pulsesapplied to the selected sites), and determining an optimal AVD for eachof these configurations. For these reasons, a large number ofcombinations is tested rendering the procedure lengthy and difficult tomanage, thus excluding it from being a routine operation.

These implantable devices ensure a joint and permanent pacing of theleft and right ventricles to resynchronize them by a technique known asCardiac Resynchronization Therapy (“CRT”) or Bi-Ventricular Pacing(“BVP”). These particular devices are commonly referred to as CRTpacemakers or CRT devices. In addition to setting an appropriate AVD,these devices optimize a delay called interventricular delay (“VVD”).The VVD is used to separate the respective moments of stimulation of theleft and the right ventricles. The VVD is adjusted to resynchronize thecontraction of the ventricles with a fine optimization of the patient'shemodynamic status. The search for an optimum pair (or couple) of {AVD,VVD} can therefore be very long, because it requires multiple scans andtests of the AVD for various potential values of VVD.

EP2 070 562 A1 and its counterpart US Patent Publication 2009/0157134(both assigned to Sorin CRM S.A.S, previously known as ELA Medical)describe a technique for testing a biventricular stimulation device bysuccessive scans of a plurality of stimulation configurations.

There remains, however, a need for a technique to evaluate in a simple,rapid, automated, and precise procedure the impact of differentstimulation parameters, including the AVD, so as to optimize thepatient's hemodynamic status.

One automated method for optimizing the AVD is described in the articleby J M Dupuis et al.: Programming Optimal Atrioventricular Delay in DualChamber Pacing Using Peak Endocardial Acceleration: Comparison with aStandard Echocardiographic Procedure, PACE 2003; 26: [Pt II], 210-213.This technique involves scanning the AVD in a given stimulationconfiguration and tracing a characteristic value of the peak ofendocardial acceleration (“PEA”) according to the AVD. The optimal valueof the AVD is considered to be the inflection point of thecharacteristics, i.e., the point corresponding to the maximum durationof ventricular filling without truncating the A wave (i.e., the minimumdelay between the closing of the mitral valve and the beginning of theQRS complex).

Although the corresponding algorithm gives satisfactory results, itrequires several minutes, especially in case of a multisite device or aCRT device that requires multiple scans of AVD for various values ofother parameters that are tested separately (including the VVD) toselect an optimal pair of {AVD, VVD}.

Another optimization technique, which is much faster, and thus can beimplemented in real time, is described in WO 2006/090397 A2 and WO2006/126185 A2. The optimization algorithm described therein uses aspike-type neural network to identify the maximum of a hemodynamicfunction (e.g., stroke volume). The spike neural network, however,requires a dedicated processor, thus redesigning of the device demandinghigher power consumption. A software implementation of the optimizationalgorithm is possible, but it requires extra computing resources thatare unattainable in an ultra-low power consumption microcontroller thatis adequate for use in an implantable medical device.

WO 2008/010220 describes yet another technique, in which a spike neuralprocessor is combined with a reinforced learning algorithm (e.g.,Q-learning), which learns and associates the cardiac conditions of thepatient with the optimal delays. The Q-learning algorithm offersimproved immunity to noise and increases the speed of convergence insearching optimal parameters. However, in order to achieve the desiredperformance, additional hardware resources are required, including amicroprocessor in addition to the spike neural processor, which incursextra cost, higher power consumption, and an increase spatialrequirement for the implantable device.

SUMMARY

It is, therefore, an objective of the present invention to provide a newsimple, rapid, automated and reliable technique for optimizing the AVDparameter.

It is further an objective of the present invention to provide such anoptimization technique for real time application, with a response timeof only a few cardiac cycles, while requiring only simple hardware andsoftware resources to be compatible with a currently availableimplantable device including a CRT pacemaker.

To this end, one aspect of the present invention is directed to anactive medical device of a known type, comprising: an implantablecardiac prosthesis device such as a pacemaker, a CRT pacemaker, adefibrillator and/or a resynchronizer. The active medical devicecomprises means for detecting atrial and ventricular events, means forstimulating the ventricle(s), and means for applying to the means forstimulating an atrio-ventricular delay AVD. The AVD is counted from themoment of detection of a spontaneous or paced atrial event. At the endof the AVD, a ventricular pacing is applied in the absence of acorresponding spontaneous ventricular event. The active medical devicefurther comprises a hemodynamic sensor; means for analyzing the signaldelivered by the hemodynamic sensor and delivering as a function of theAVD a hemodynamic parameter having a variation that is represented by asigmoid-type characteristic; and means for seeking an optimal AVD byanalyzing the sigmoid-type characteristic.

In one embodiment of the present invention, the means for seeking anoptimal AVD preferably operates without scanning of the sigmoid-typecharacteristic, and more preferably comprises: means for applying areference AVD (Xc) determined by a current value of the optimal AVD orby a default AVD value, e.g., a predetermined value of AVD, andmeasuring the corresponding hemodynamic parameter; means for applying atleast one left AVD that is less than the reference AVD, and measuringthe corresponding hemodynamic parameter; means for applying at least oneright AVD that is greater than the reference AVD, and measuring thecorresponding hemodynamic parameter; means for evaluating the secondderivative value of the function at the respective points of thecharacteristic corresponding to the reference AVD, the left AVD and theright AVD; means for estimating, from the second derivative values thusobtained, the position of an intermediate point of the characteristicfor which the second derivative value is equal to zero or at a minimum;and means for determining the corresponding AVD for that intermediatepoint as the optimal AVD.

In one embodiment, the value of the reference AVD is selected as eithera current value of the optimal AVD or a predetermined AVD value.

In another embodiment, the device in accordance with the presentinvention comprises: means for applying two left AVDs having twodifferent time intervals, respectively less than the reference AVD;means for applying two right AVDs having two different time intervals,respectively greater than the reference AVD. In this embodiment: themeans for estimating the second derivative of the function at the pointcorresponding to the reference AVD further comprises means operating bya computation of the increments of the function between the left AVD andthe right AVD, framing the reference AVD; the means estimating thesecond derivative of the function at the point corresponding to the leftAVD is means operating by computation of the increments between the twoleft AVDs; and the means for estimating the second derivative of thefunction at the point corresponding to right AVD is means operating bycomputation of the increments between the two right AVDs.

In one preferred embodiment, the consecutive values of left AVD, rightAVD and reference AVD are separated by equal time steps.

In another embodiment, the means for estimating the position of theintermediate point comprises means for determining an existence of azero crossing between the second derivative values corresponding to theleft AVD and right AVD, and means for, in this case, operating a linearinterpolation calculation between the points corresponding to the leftAVD and the right AVD.

Preferably, the means for estimating the position of the intermediatepoint include means for determining an absence of zero crossing betweenthe second derivative values corresponding to the left AVD and to theright AVD, and means for, in this case, select as an intermediate point,among the three points corresponding to the reference AVD, to the leftAVD and to the right AVD, the one minimizing the second derivativevalue.

In yet another embodiment, the device preferably includes means fordetermining the sign of the slope of the function at the characteristicpoint corresponding to the reference AVD, and means for inhibiting themeans for searching for an optimal AVD on detection of a non-compliantslope sign.

In a preferred embodiment, the device further comprises means foranalyzing the activity status of the patient and conditioning theexecution of the means for searching for an optimal AVD on a detectionof a change in the patient activity. Preferably, the means for analyzingthe state of activity comprises means for comparing a heart rate meanvalue over a short-term and a heart rate mean value over a long-term,and means for activating the means for searching for an optimum AVD ifthe difference between these two heart rate mean values exceeds apredetermined threshold.

In one embodiment, the hemodynamic sensor is an endocardial accelerationEA sensor, and the hemodynamic parameter is a peak value of endocardialacceleration PEA, derived from either or both of the two peaks ofacceleration endocardial PEA1 appearing during the ventricularisovolumetric contraction phase and acceleration endocardial PEA2appearing during the phase of isovolumetric ventricular relaxation.

One embodiment of the present disclosure relates to a method for use byan active medical device. The method includes using a stimulation deviceand an endocardial acceleration sensor to obtain a plurality ofhemodynamic parameters associated with at least three atrioventriculardelays. The method further includes using the plurality of hemodynamicparameters to find a second derivative associated with theatrioventricular delays. The method further includes using interpolationto estimate an atrioventricular delay which will reduce the secondderivative associated with the atrioventricular delays. The methodfurther includes using the estimated atrioventricular delay in asubsequent stimulation.

Another embodiment of the present disclosure relates to a system fordetermining an optimal atrioventricular delay following delivery of astimulation pulse to the heart of a patient. The system includes agenerator capable of delivering a stimulation pulse to the heart of thepatient. The system further includes a hemodynamic sensor capable ofmeasuring a hemodynamic signal representing cardiac output from theheart corresponding to an atrioventricular delay. The system furtherincludes an acquisition circuit for receiving the hemodynamic signalrepresenting cardiac output from the hemodynamic sensor and deliveringthe hemodynamic signal to a controller. The system further includes thecontroller configured to control the operation of the generator based ona plurality of hemodynamic signals received from the acquisitioncircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentinvention will become apparent to a person of ordinary skill in the artfrom the following detailed description of embodiments of the presentinvention, made with reference to the drawings annexed, in which likereference characters refer to like elements and in which:

FIG. 1 is a system block diagram of a closed-loop, real-time hemodynamicCRT device, according to one embodiment;

FIG. 2 illustrates exemplary variation of the value of the peak ofendocardial acceleration (PEA) obtained from a hemodynamic sensor as afunction of the AVD;

FIG. 3 illustrates measured PEA values in accordance with varying AVD,and a procedure to derive an optimal AVD value;

FIG. 4 is a flow chart of an algorithm for detecting a change in apatient's hemodynamic status; and

FIG. 5 is a representation of the evolution of a patient's long-term andshort-term heart rate mean values during successive cardiac cycles ofthe heart rate, and the difference between these two mean valuescompared to an effort state detection threshold.

DETAILED DESCRIPTION

With reference to FIGS. 1-5, various embodiments of the presentinvention will now be described.

According to one embodiment, the present technique for optimizing AVD isimplemented in a CRT device, i.e., a resynchronizer device that ensuresjoint and permanent stimulation of the two ventricles, so as toresynchronize them. The present invention is advantageously applied tothis type of device to jointly adjust several interdependent parametersincluding the AVD and WD delays. However, it should be understood thatthe present invention is not limited to such CRT devices and can beapplied to optimize the value of the AVD at rest or during exercise in“dual chamber” devices and any other device including more complexdevices (triple chamber, quadruple chamber, multisite, CRT, . . . ) aswell as the defibrillators including pacing capabilities.

As regards to its software aspects, the present invention can beimplemented by an appropriate programming of the controlling software ofa known device, for example, a cardiac pacemaker or adefibrillator/cardioverter, including means for collecting a signalprovided by endocardial leads and/or one or more implanted sensors. Asregards to it hardware aspects, the present invention can be implementedin a device that includes programmable microcontroller and/ormicroprocessor circuitry to receive, format, and process electricalsignals collected (detected) by one or more implanted electrodes, anddeliver stimulation pulses to these electrodes. It is possible totransmit by telemetry software and store it in a memory of theimplantable device to execute the functions of the present invention asdescribed herein. The adaptation of these devices to implement thefunctions and features of the present invention is believed to be withinthe abilities of a person of ordinary skill in the art, and thereforewill not be described in detail. One suitable type of device to whichthe present invention may particularly be applied are those of the Replyand Paradym device families produced and marketed by Sorin CRM, ClamartFrance, formerly known as ELA Medical, Montrouge, France.

General Configuration of the Device

The various elements involved in controlling the AVD (and the VVD, inthe case of a CRT device) are illustrated in FIG. 1, in the form ofschematic functional blocks. The reference 10 denotes the generator ofthe device connected to the heart via leads to collect myocardiumdepolarization signals and to stimulate the myocardium by deliveringelectrical pulses to the different cavities of the heart. A lead 12implanted in the right atrium (RA) and a lead 14 implanted in the rightventricle (RV) allow optimizing the atrioventricular delay AVD betweenthe instants of stimulation of the atrium and of the right ventricle fora CRT device. A lead 16 implanted in the vicinity of the left ventricle(LV) in combination with the lead 14 implanted in the right ventricle,allows optimizing the interventricular delay VVD between the left andright ventricles.

The hemodynamic sensor 18 measures hemodynamic signals representingcardiac output from the heart. More specifically, the hemodynamic sensor18 estimates changes in contractility correlated with increases in bloodpressure. Hemodynamic sensors differ from activity sensors (e.g.,acceleration sensors) or metabolic sensors (e.g., minute ventilationsensors) that are intended only to diagnose the presence or level of anactivity by the patient and to quantify the patient's metabolic needs.Depending on the patient's level of activity or metabolic needs, thestimulation heart rate is adapted. However, the hemodynamic sensor 18not only monitors the patient's efforts as does an activity sensor or ametabolic sensor, but also provides an indication of the patient'shemodynamic tolerance in relation to certain events, especially thetolerance to a change in the AVD (and VVD, if applicable) parameters bythe device.

In a preferred embodiment, the hemodynamic sensor 18 is an endocardialacceleration sensor, called “PEA sensor,” having an outputrepresentative of the peak of endocardial acceleration. For variousdescriptions of such a PEA sensor, reference is made to EP 0515319 A1(assigned to Sorin Biomedica Cardio SpA), which describes how to collectan endocardial acceleration signal (EA signal) using an endocardial leadprovided with a distal electrode of stimulation located at the apex ofthe ventricle and incorporating a micro-accelerometer to measure theendocardial acceleration, and EP 0655260 A1 (assigned to Sorin BiomedicaCardio SpA), which describes a method for processing the signal ofmeasured endocardial acceleration to derive a particular value of thepeaks of endocardial acceleration corresponding to the two major noisesthat are recognizable in each cycle of a healthy heart.

Suitable PEA sensors have been developed by the assignee hereof andcommercialized under the brand SonR Fix (trademark) for the family ofdefibrillators products known by the brand Paradym and Paradym CRTdevices.

The present invention also may be implemented in a configuration inwhich the PEA signal is noninvasively collected by an external sensor,rather than by an implanted sensor, for example, by means of anaccelerometer sensor attached to the patient's chest at the sternum.

Whether implanted or external, hemodynamic sensor 18 delivers a signalrepresenting the patient's cardiac output to an acquisition circuit 20.The acquisition circuit 20 is preferably, but not necessarily,incorporated into the generator 10 of the implanted device, but it alsomay be externally located outside the patient's body.

The acquisition circuit 20 delivers a PEA hemodynamic signal to acontroller 22 which forms a closed-loop system. The transmission of aPEA hemodynamic signal can be direct (in case of a sensor 18 and circuit20 being internal to the device), or be performed by telemetry (in caseof an external hemodynamic sensor 18 and a controller 22 incorporatedinto an implanted device or, conversely, in case of a hemodynamic sensor18 and an external controller 22 that is integrated in an externalprogrammer used for setting up the generator during a visit to apractitioner).

The controller 22 implements a closed-loop algorithm to derive optimalvalues of the parameters for controlling the generator, including anoptimal AVD, AVD_(opt) (and an optimal VVD for a CRT device). The systemmay also include (but not necessarily) an auxiliary sensor 24 formeasuring a physical parameter, such as an accelerometer or “G Sensor”to detect a beginning or an end of a patient's activity.

AVD Optimization Algorithm

The acquisition circuit 20 delivers signals representative of the peakof endocardial acceleration PEA, more precisely, the first peak ofendocardial acceleration (“PEA1”) that corresponds to the closure of themitral and tricuspid valves, at the beginning of the ventricularisovolumetric contraction phase (systole). The variations of PEA1 areclosely linked to changes in pressure in the ventricle, thereforerepresenting the myocardial contractility. The amplitude of the peakPEA1 is particularly correlated to the positive maximum of the pressurevariation dP/dt in the left ventricle.

The variations of PEA as a function of the AVD follow a relation Z=f(AVD) generally represented by the characteristic curve shown in FIG. 2.The optimum value of the AVD is the point AVO_(opt) located at theinflection point of this characteristic curve. The inflection pointcorresponds to a maximum duration of ventricular filling withouttruncation of the A wave (i.e., a minimum delay between the closing ofthe mitral valve and the beginning of the QRS wave).

In the vicinity of the inflection point representative of an optimalAVD, the characteristic curve can be approximated by a sigmoid functionthat contains exponential terms: if X denotes the current value of theAVD and X_(o) denotes the value of an optimal AVD, the shape of thesigmoid curve that reflects the variations of the signal representativeof PEA in the vicinity of the optimum sought corresponds to a functionof the following type:

$Y = {a + {b\left( \frac{1}{1 + {\exp\left( {c\left( {X - X_{0}} \right)} \right)}} \right)}}$in which the parameters a, b, c and X_(o) are quantities that vary overtime and according to the patient's status.

The second derivative at point X_(o), a point of inflection of thesigmoid curve Y, is equal or is close to zero, and is used as the basisto optimize the AVD.

The details of the optimization algorithm of the present invention willnow be further explained with reference to FIG. 3. The optimizationalgorithm starts with the last optimal value of the AVD, hereinafterdesignated X_(c) (i.e., the central value of the current value X of theAVD) and searches for the point at which the second derivative is zeroor minimal around this value X_(c). In the case that no value of optimalAVD is known, especially at the start of the algorithm, a typical meanvalue may be chosen, for example X_(c)=120 ms.

The algorithm tests several points surrounding delay X_(c), and from theresults of these tests, determines by interpolation or by a search for aminimum, the value of an optimal AVD. In other words, instead ofperforming continuous scans of the values of the AVD to perform theoptimization, tests are performed on only a limited number of discretevalues of the AVD. The algorithm directly deduces an optimum positionfrom the results of these tests, regardless of the order of the testsunlike continuous scans that assume a continuous variation of the AVD ona relatively wide range.

In addition to the central point X_(c) that corresponds to the lastknown optimal AVD, measurements also are performed for two points X_(L)and X_(LL) located to the left of X_(c), and for two points X_(R) andX_(RR) located on the right of that point X_(c) of the characteristic.These various points X_(LL), X_(L), X_(c), X_(R) and X_(RR) areseparated by corresponding delays D₁, D₂, D₃ and D₄. Preferably, thedelays D₁, D₂, D₃ and D₄ are equal and correspond to the step D forscanning the AVD, for example, D=20 ms. In reference to the centraldelay X_(c), the values of the delays X_(L), X_(R), X_(RR) and X_(LL)are given by:X _(L) =X _(C) −D.(LeftAVD)X _(R) =X _(C) +D,(RightAVD)X _(LL) =X _(L) −DX _(RR) =X _(R) +D

In case variable steps of delays are used, the five points are chosensuch that:X _(L) =X _(C) −D2X _(R) =X _(C) +D3X _(LL) =X _(L) −D1X _(RR) =X _(R) +D4Y₁, Y₂, Y₃, Y₄ and Y₅ are the five measurements of PEA corresponding tothe respective values X_(LL), X_(L), X_(C), X_(R), X_(RR) of the AVD.Each value Y₁ to Y₅ is preferably obtained by averaging from four to tenmeasurements of the PEA.

The algorithm then calculates:

the second derivative at the center E_(C), at the point (X_(C), Y₃)

the second left derivative E_(L), at the point (X_(L), Y₂), and

the second right derivative E_(R), at the point (X_(R), Y₄),

and the following second derivative values are obtained by:

$E_{C} = \left( \frac{2\left( {{\left( {D_{2} + D_{3}} \right) \cdot Y_{3}} - {D_{3} \cdot Y_{2}} - {D_{1} \cdot Y_{4}}} \right)}{\left( {D_{2} + D_{3}} \right)} \right)$$E_{L} = \left( \frac{2\left( {{\left( {D_{1} + D_{2}} \right) \cdot Y_{2}} - {D_{2} \cdot Y_{1}} - {D_{1} \cdot Y_{3}}} \right)}{\left( {D_{1} + D_{2}} \right)} \right)$$E_{R} = \left( \frac{2\left( {{\left( {D_{3} + D_{4}} \right) \cdot Y_{4}} - {D_{3} \cdot Y_{3}} - {D_{1} \cdot Y_{5}}} \right)}{\left( {D_{3} + D_{4}} \right)} \right)$

When the step D is fixed, the calculation is simplified and the secondderivative values are obtained by:E _(C)=2·Y ₃ −Y ₂ −Y ₄E _(L)=2·Y ₂ −Y ₁ −Y ₃E _(R)=2·Y ₄ −Y ₃ −Y ₅

It is noted that the values X and Y are digitized sampled values, andthe calculation of the second derivative is reduced to a simplearithmetic calculation.

Once the three second derivatives E_(L), E_(C) and E_(R) are calculated,the AVD that corresponds to a zero crossing of the second derivative issought, by applying the three rules (1), (2) and (3) as below:

If ((E_(L)>0) and (E_(C)<0)) then

-   -   Rule (1)        The zero crossing is calculated by linear interpolation:        AVD_(opt) =X _(L) +E _(L)*(X _(C) −X _(L))/(E _(L) −E _(C))        Optimum found→End of the algorithm        If ((E_(C)>0) and (E_(R)<0)) then    -   Rule (2)        The zero crossing is calculated by linear interpolation:        AVD_(opt) =X _(C) +E _(C)*(X _(R) −X _(C))/(E _(C) −E _(R))        Optimum found→End of the algorithm        Otherwise    -   Rule (3)

If there is no zero crossing of the second derivative X_(L) and X_(R),the optimum is approximated by the point at which the value of thesecond derivative is minimal (i.e., the nearest value to zero) inabsolute value. The determination according to this rule is made by thefollowing steps:

-   -   AVD_(opt)=X_(L), E_(m)=Abs(E_(L));    -   If (Abs(E_(C))<E_(m)) then ADV_(opt)=X_(C), E_(m)=Abs(E_(C));    -   If (Abs(E_(R))<E_(m)) then ADV_(opt)=X_(R);    -   The optimum is approached→End of the algorithm.

At the end of the algorithm, one of three rules was necessarily applied,and the optimal value AVD_(opt) is found to be between X_(L) and X_(R).

For security purpose, there may be an additional condition applied toprevent modification of the AVD in cases where the PEA measurementscannot be fitted into a decreasing sigmoid characteristic curve in thedirection of increasing AVD. In one embodiment, an additional conditionof Y₂>Y₄ is applied. This condition is equivalent to testing whether theslope of the curve PEA=f(AVD) around the central point X_(C) isnegative.

Detection of Patient Activity

According to another aspect of the present invention, the optimizationalgorithm of the AVD is not applied in a continuous manner, but only isapplied upon detection of a significant change in the patient'sactivity. For example, such a significant change in the patient'sactivity occurs in a detected transition from a rest condition to anexercise condition, or vice versa. The optimization algorithm is appliedto adapt or closed-loop control the AVD if a continuous improvement ofthe patient's hemodynamic status is sought.

A preferred algorithm adapted for detecting a change in the patient'shemodynamic status by passing from a rest state to an exercise state orvice versa is described with reference to FIGS. 4 and 5. As illustratedin the flowchart of FIG. 4, the heart rate value HR(k) at each cardiaccycle k is constantly collected (block 26). A short-term mean heart ratevalue HR₁₀ and a long-term mean heart rate value HR₁₀₀, respectivelydetermined over the last ten and hundred cardiac cycles, are calculatedor updated (block 28), using a recursive formula without saving the last10 or 100 heart rate values in the memory, by:H ₁₀=(1−1/10)HR₁₀+HR(k)/10HR₁₀₀=(1−1/100)HR₁₀₀+HR(k)/100

The next step (block 30) is to test for a rapid change in the mean heartrate values that is representative of a change in the level of patient'sactivity. To that purpose, the difference |HR₁₀₀−HR₁₀| between the meanvalues HR₁₀₀ and HR₁₀ is calculated, and the absolute value of thedifference is compared to a threshold S (test 30). If the condition|HR₁₀₀−HR₁₀|>S is satisfied, then the search algorithm of a new optimumAVD is triggered (block 32).

It is noted that, alternatively or in addition, the threshold test 30leading to the triggering of the optimization algorithm can beconditioned on another technique for detecting the patient's effort,including using a signal delivered by an accelerometer G (e.g., sensor24 in FIG. 1).

With reference to FIG. 5, the variations of the three parameters HR(k),HR₁₀ and HR₁₀₀ are illustrated over time, with an example of a suddenincrease in the level of effort, starting from the patient's cardiaccycle No. 200. This sudden change is detected, as shown in the timingscale at the bottom of FIG. 5, by the crossing of the threshold by thedifference S<|HR₁₀₀−HR₁₀|, which triggers an adaption of the AVD duringexercise with a very low delay.

One skilled in the art will appreciate that the present invention may bepracticed by other than the embodiments described above, which arepresented for purposes of illustration and not of limitation.

The invention claimed is:
 1. A method for use by an active medicaldevice, comprising: using a stimulation device and an endocardialacceleration sensor to obtain a plurality of hemodynamic parametersassociated with at least three atrioventricular delays; using theplurality of hemodynamic parameters to find a second derivativeassociated with each of the atrioventricular delays; calculating anatrioventricular delay that corresponds to a minimum absolute value ofthe second derivatives; and using the calculated atrioventricular delayin a subsequent stimulation.
 2. The method of claim 1, wherein using thestimulation device and the endocardial acceleration sensor to obtain aplurality of hemodynamic parameters associated with at least threeatrioventricular delays further comprises: selecting a referenceatrioventricular delay; applying the reference atrioventricular delay;applying at least one left atrioventricular delay that is less than thereference atrioventricular delay; and applying at least one rightatrioventricular delay that is greater than the referenceatrioventricular delay.
 3. The method of claim 2, wherein the referenceatrioventricular delay is an estimated atrioventricular delay.
 4. Themethod of claim 2, wherein the reference atrioventricular delay is apredetermined atrioventricular delay.
 5. The method of claim 1, whereinthe at least three atrioventricular delays are separated by equal timeintervals.
 6. The method of claim 1, wherein the plurality ofhemodynamic parameters comprises a peak value of endocardialacceleration associated with each atrioventricular delay.
 7. The methodof claim 6, wherein the peak value of endocardial acceleration isdetermined as the average value of at least two measurements of the peakvalue of endocardial acceleration.
 8. The method of claim 2, furthercomprising: calculating a left second derivative at an atrioventriculardelay less than the reference atrioventricular delay, a reference secondderivative at the reference atrioventricular delay, and a right secondderivative at an atrioventricular delay greater than the referenceatrioventricular delay; and calculating the atrioventricular delay thatcorresponds to the minimum absolute value of the second derivatives,wherein the minimum is determined based on: (a) linear interpolationbetween the left second derivative and the reference second derivative,if the left second derivative is greater than zero and the referencesecond derivative is less than or equal to zero; (b) linearinterpolation between the reference second derivative and the rightsecond derivative, if the reference second derivative is greater thanzero and the right second derivative is less than or equal to zero; or(c) the second derivative that has the lowest absolute value fromamongst the left second derivative, the reference second derivative, andthe right second derivative.
 9. The method of claim 1, furthercomprising initiating the method in response to a change in activity ofa patient.
 10. The method of claim 9, further comprising detecting achange in the hemodynamic status of the patient.
 11. The method of claim10, further comprising: calculating a short-term mean heart rate value;calculating a long-term mean heart rate value; and initiating the methodif the absolute value of the difference between the short-term meanheart rate value and the long-term mean heart rate value exceeds athreshold value.
 12. The method of claim 9, further comprising using anaccelerometer to detect the change in activity of the patient.
 13. Asystem for determining an atrioventricular delay following delivery of astimulation pulse to the heart of a patient, comprising: a generator fordelivering a stimulation pulse to the heart of the patient; ahemodynamic sensor configured to measure a hemodynamic signalrepresenting cardiac output from the heart corresponding to anatrioventricular delay and configured to obtain a plurality ofhemodynamic parameters associated with the atrioventricular delay; and acontroller for receiving the hemodynamic signal representing cardiacoutput from the hemodynamic sensor; wherein the controller is furtherconfigured to: control the generator to apply at least threeatrioventricular delays; use the plurality of hemodynamic parameters tofind a second derivative associated with each of the at least threeatrioventricular delays; calculate an atrioventricular delay thatcorresponds to a minimum absolute value of the second derivatives;control the operation of the generator to apply the estimatedatrioventricular delay in a subsequent stimulation.
 14. The system ofclaim 13, wherein the hemodynamic sensor is an endocardial accelerationsensor capable of measuring a peak value of endocardial acceleration,and the hemodynamic signal delivered by the acquisition circuit is ameasure of the peak value of endocardial acceleration.
 15. The system ofclaim 14, wherein the peak value of endocardial acceleration isdetermined as the average value of at least two measurements of the peakvalue of endocardial acceleration.
 16. The system of claim 13, whereinthe controller is further configured to: control the generator to applyat least three atrioventricular delays, comprising a referenceatrioventricular delay, a left atrioventricular delay that is less thanthe reference atrioventricular delay, and a right atrioventricular delaythat is greater than the reference atrioventricular delay.
 17. Thesystem of claim 16, wherein the reference atrioventricular delay is anestimated atrioventricular delay.
 18. The system of claim 16, whereinthe reference atrioventricular delay is a predetermined atrioventriculardelay.
 19. The system of claim 13, wherein the plurality ofatrioventricular delays are separated by equal time intervals.
 20. Thesystem of claim 16, wherein the controller is further configured to:calculate a left second derivative at the left atrioventricular delay, areference second derivative at the reference atrioventricular delay, anda right second derivative at the right atrioventricular delay; andcalculate the atrioventricular delay that corresponds to the minimumabsolute value of the second derivatives, wherein the minimum isdetermined based on: (a) linear interpolation between the left secondderivative and the reference second derivative, if the left secondderivative is greater than zero and the reference second derivative isless than or equal to zero; (b) linear interpolation between thereference second derivative and the right second derivative, if thereference second derivative is greater than zero and the right secondderivative is less than or equal to zero; or (c) the second derivativehaving the lowest absolute value.